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* feat: total wants gauge

* fix: in gauges count wants regardless of which peers they're sent to

* fix: want block gauge calculation

* refactor: simplify peermanagerwants

Instead of tracking offsets, just create a "new" slice starting with the
broadcast cancel slice. Under the covers, this will just use the same memory
over and over.

* feat: small optimizations

* feat: move broadcast wantlist into the peermanager

This deduplicates some state and allows us to do less book-keeping for broadcast
wants. We should probably rename the PeerManager to the WantManager and rename the
WantManager to something else.

* fix: lint warnings

* refactor: add reverse index to peerWantManager to speed up cancels

* refactor: in peerWantManager use ForEach instead of allocating lists

This makes it easier to tell where module boundaries are.

And expose a separate function for _just_ getting want-blocks. When the user
runs `ipfs bitswap wantlist`, they expect to see everything the node is
currently looking for.
Co-Authored-By: dirkmc <dirkmdev@gmail.com>

This makes reading the docs much easier as it's clear what's "private" and
what's not.

fixes #238

This commit extends the bitswap protocol with two additional wantlist properties:

* WANT_HAVE/HAVE: Instead of asking for a block, a node can specify that they
  want to know if any peers "have" the block.
* WANT_HAVE_NOT/HAVE_NOT: Instead of waiting for a timeout, a node can explicitly
  request to be told immediately if their peers don't currently have the given
  block.

Additionally, nodes now tell their peers how much data they have queued to send
them when sending messages. This allows peers to better distribute requests,
keeping all peers busy but not overloaded.

Changes in this PR are described in: https://github.com/ipfs/go-bitswap/issues/186